1. Field of the Invention
The present invention relates to a liquid sealed vibration isolating device for use in an engine mount or the like for motor vehicles and more particularly, to the device capable of effectively reducing noises created due to a cavitation phenomenon.
2. Description of the Related Art
In the liquid sealed vibration isolating device of this kind, negative pressure is developed instantaneously in a primary liquid chamber when the heavy load is applied, so as to cause the cavitation phenomenon that hydraulic liquid is partially vaporized, and then the noise is generated. In view of this, there are proposed various kinds of devices for reducing transmission of the noise. As one of the proposed devices, there is known the art that in the case of where the primary liquid chamber turns the negative pressure, the primary liquid chamber and an orifice passage are short circuited to leak the hydraulic liquid (see a patent reference 1). Also, there is known the art that an elastic diaphragm is arranged in a through hole provided in a partition member in such a manner that an outer peripheral portion of the diaphragm can be contacted with and be separated from an inner surface of the through hole. When vibration of large amplitude is inputted, an outer peripheral portion of the elastic diaphragm is deformed to project into a secondary liquid chamber so as to form a clearance for having a large amount of the hydraulic liquid leaked (see a patent reference 2).    Patent reference 1: Japanese patent laid open publication No. 2003-148548.    Patent reference 2: Japanese patent laid-open publication No. 2006-132615.
In the short circuit structure of the above patent reference 1, a portion of a wall constituting the orifice passage is formed by a lid portion of an elastic body. When the negative pressure is produced in the primary liquid chamber, the lid portion is elastically deformed to short circuit the orifice passage and the primary liquid chamber. Therefore, since the flow of the hydraulic liquid at the time of relief is throttled by a communication port of the orifice passage located on the side of the secondary liquid chamber, an enough relief amount may not necessarily by obtained instantaneously. Moreover, since the lid portion is formed integral with an insulator of a vibration isolating subject, the lid portion is deformed together with the insulator without being limited to the relief time thereby being capable of influencing the orifice performance. Accordingly, the relief valve structure in which the orifice passage is not involved is required.
Further, in the structure of the patent reference 2 employing the elastic diaphragm, a leak passage can be provided without relation to the orifice passage. However, since the elastic diaphragm itself is provided for the purpose of repeating the elastic deformation by the fluctuation of an internal pressure of the liquid chamber, the elastic diaphragm is elastically deformed also in the case where the vibration of minute amplitude is inputted. Then, the seal of the outer peripheral portion works incompletely to cause the leakage at the time of pressurization, so as to decrease the damping performance. Therefore, it is required to prevent the leakage by the minute vibration while controlling the generation of the cavitation phenomenon, thereby to minimize the decrease in the damping performance.
Further, it is ascertained that the elastic diaphragm is rotatively moved by the continuous input of the vibration of large amplitude thereby to cause the influence on the prevention of the cavitation phenomenon by the positional slippage between the relief valve and a leak passage. Therefore, it became necessary to arrest the rotation of the elastic diaphragm.
Accordingly, the present invention has its object to provide a liquid sealed vibration isolating device which materializes the above mentioned requirements.